Thermal switch



July 20, 1943. c c 2,324,906

THERMAL SWITCH Filed March 24, 1941 Inverfior: BasiL NoeL CLack,

His Ai torneg.

Patented July 20, 1943 THERMAL SWITCH Basil Noel Clack, Wemble General Electric Com New York Application March 24 In Great Brit 6 Claims.

The present invention relates to electric dis charge devices of the cathode glow type wherein it is important that the starting potential should be substantially constant and, in particular, should not rise above some assigned limit.

The invention relates particularly'to thermally operated switches comprising a thermally responsive element, such as a bimetallic strip, which is heated by a cathode glow discharge. Switches of this type are now used commercially for controlling the starting of fluorescent discharge lamps having electrodes which are heated to their operating temperature before a discharge is started therebetween. In one type of commercial circuit the switch is connected in parallel with the lamp and connects the electrodes, or the heaters for the electrodes, of the lamp in series across the terminals of a current source to preheat said electrodes to their operating temperature and breaks the series connection therebetween when the electrodes have attained such temperature. A choke-coil is connected in the circuit between one of said electrodes and a terminal of the current source. Said choke-coil ballasts the arc and provides an inductive voltage surge for starting the discharge in the lamp when the series connection between the lamp electrodes is broken by the switch.

In such circuits a cathode glow discharge starts in the switch when the potential is first applied. The heat from the glow discharge causes the bimetallic strip to bend and thus close the switch contacts. This connects the electrodes of the lamp in series and shunts the glow discharge path in the switch. After a time sufiicient to raise the lamp electrodes to their operating temperature, the switch contacts separate due to the cooling of the bimetallic elements, but the glow discharge does not start again in the switch if the discharge in the lamp has started because the voltage across the switch is then not the full supply voltage, but the voltage across the lamp. It is apparent that in such circuits the starting potential of the switch should be less than the supply voltage, but greater than the operating voltage of the lamp.

I have observed that switches of the kind described above, which are perfectly satisfactory for use in such lamp circuits when the switch is exposed to light, are apt to develop an abnormally high starting potential when they have been in the dark for some time. Frequently the starting potential of the switch is then higher than the supply voltage and, of course, the switch y, England, assignor to pany, a corporation of 1941, Serial No. 385,003 ain April 15, 1940 is useless for the purpose of starting the lamp in such circuits.

The object of the invention is to provide a thermal switch of the cathode glow discharge type having substantially the same starting potential in the light and in the dark. Still further objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description and from the appended claims.

I have discovered and demonstrated that thermal switches of the cathode glow type comprising a member of uranium glass have substantially the same starting potential in the light and in the dark. I have demonstrated further that the starting potential of such switches including a uranium glass member is substantially the same in the dark as it was in the light even after the switch has been in complete darkness for a week without being operated.

In the drawing accompanying and forming part of this specification, one species of a thermal glow switch having substantially the same starting potential in the dark and in the light is shown in a side, elevational view.

Referring to the drawing, the thermal switch illustrated comprises a sealed, tubular envelope 1 of glass having a glass stem 6 fused thereto. Said envelope I is about 50 mm. in length and about 18 mm. in internal diameter and has a gaseous atmosphere therein consisting of helium mixed with approximately 0.15 per cent of argon at a pressure of approximately 45 mm. mercury column. Two bimetallic strips 2 and 3 having contacts 4 and 5 mounted at the end thereof remote from the stem 6 are mounted in parallel positions in said envelope I. Said strip 2 is fastened, as by welding, to the current inlead wire I and the strip 3 is similarly mounted on the current inlead wire 8. Said wires 1 and 8 pass through and make a hermetic joint with the stem 6. The contacts 4 and 5 are separated a distance of approximately 5 mm. when the strips 2 and 3 are cold and engage each other when the strips are heated by the cathode glow discharge therebetween. Switches having this structure and an envelope of any of the uranium free, commercial envelope glasses, such as the lead or lime glasses, have a starting potential in the light of about volts. The starting potential rises considerably in the dark and is frequently higher than the supply voltage. When the envelope I and the stem 6 consist of uranium glass, however, the starting potential is approximately 160 volts in the dark even after the switch has been in complete darkness for a week. The switch is thus highly useful in fluorescent discharge lamp circuits.

A uranium glass having the following batch composition gives good results:

Sand 140 lbs. Sodium carbonate 62 lbs. lviagnesite '7 lbs. Potash feldspar lbs., 4 oz. Cadmium sulphide 4 lbs., 3 oz. Uranium yellow (sodium biuranate) 3 lbs., 10 oz.

While I have shown and described the invention in connection with one species of a thermal switch, it will be understood, of course. that I contemplate its use in switches having difierent structure, for example, switches similar to those disclosed in the co-pending applications of Wayne C. Smitley, Serial No. 289,897, filed August 12, 1939, and Serial No. 295,801, filed September 20, 1939. Both of said applications are assigned to the assignee of the present application. The invention is also useful in thermal switches having electron emissive materials such as barium, strontium, calcium, zinc, magnesium, or the like, or mixtures or compounds of these materials in or on the electrodes of the switch for securing a desired starting potential. Any of the discharge conducting gases, such as neon, argon, xenon, krypton and helium, or mixtures thereof are used in the switch for the same purpose when desired.

The envelope of the switch is made of any of the uranium free glasses used commercially for discharge devices, when desired, and the uranium glass is mounted in said envelope. In such structure the uranium glass is preferably in the form of a sleeve mounted about the electrodes, though it may also be in a finely divided state and applied as a coating to the inner surface of the switch envelope. When desired, the stem 6 of the envelope I consists of one of the nonuranium containing glasses, such as Nonex, used commercially for such parts and the envelope I consists in whole or in part of uranium glass, or the envelope I consists of a uraniumfree glass and the stem 6 of uranium glass.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A glow-type thermal switch comprising a sealed envelope containing an ionizable atmosphere and electrical contacts, said switchstructure including a uranium glass member which serves to lower the starting potential of the switch in the dark.

2. A glow-type thermal switch comprising a sealed envelope containing an ionizable atmosphere and electrical contacts, at least a portion of said envelope consisting of uranium glass which serves to lower the starting potential of the switch in the dark.

3. A glow-type thermal switch comprising a sealed envelope containing an ionizable atmosphere, electrical contacts and uranium glass which serves to lower the starting potential of the switch in the dark.

4. A glow-type thermal switch comprising a sealed envelope containing helium and electrical contacts, said switch structure including a uranium glass member which serves to lower the starting potential of the switch in the dark.

5. A glow-type thermal switch comprising a sealed envelope containing electrical contacts and an atmosphere of helium mixed with approximately 0.l5 per cent of argon at a pressure of approximately mm., said switch structure including a uranium glass member which serves to lower the starting potential of the switch in the dark.

6. A glow-type thermal switch comprising a sealed, uranium glass envelope containing electrical contacts and an atmosphere of helium mixed with approximately 0.15 per cent of argon at a pressure of approximately 45 mm., said glass having the following batch composition:

Sand lbs. Sodium carbonate 62 lbs. Magnesite '7 lbs. Potash feldspar 10 lbs., 4 oz. Cadmium sulphide 4 lbs., 3 oz. Uranium yellow (sodium biuranate) 31bs., 10% oz.,

said uranium glass envelope serving to lower the starting potential of the switch in the dark.

v BASIL NOEL CLACK. 

